The term "recessed lip" or "reversed lip" as used herein refers to a generally U-shaped hollow member which is formed at the upper open edge of an article. The lip is provided to increase the rigidity and integrity of the article.
The molding of a deep recessed lip into a plastic article has traditionally been achieved through the use of injection molding. Blow molding techniques have also been used to form a recessed lip in an article. Prior blow molding apparatus typically comprise two horizontally reciprocally movable mold halves which are arranged symmetrically about a vertical plane. Each mold half is a mirror image of the other. Each mold half may also be divided by a horizontal plane into two vertically reciprocally movable sections. A mold of this type is used to simultaneously form two articles which may be separated upon formation.
The blow molding process is initiated with all the mold parts separated from one another. A parison or tube of plastic is extruded around a blow tube between the reciprocally movable mold halves. The parison is typically polyethylene, although any thermoplastic which is blow moldable may be used. A polyethylene parison is typically extruded at a temperature of about 375.degree. F. The mold halves are moved horizontally toward one another. Pinch-off edges make a seal for the parison within the mold and prepare the flash for easy removal. The parison within the mold is then inflated or pressurized with air between about 60 and 150 psi. The parison is expanded into contact with the inner surface of the mold cavity. A portion of the mold is formed radially outwardly of the mold cavity that forms the main body of the article. The parison is pushed into the interior of the outward portions and forms the lip on the article. Thereafter, the air pressure in the mold cavity may be lowered to about 5 to 40 psi.
The mold cavity is also provided with sections that move vertically toward one another so as to further define the lip. Such an apparatus is described in copending application Ser. No. 07/265,163, now U.S. Pat. No. 4,972,963. As the mold sections are pushed together, the outwardly extending portions of the lip are compressed. Further apparatus include moveable mold sections that placed the outwardly extending portions into an over lapping condition. Such an apparatus is described in U.S. Pat. No. 4,378,328.
The entire series of movements of the various mold sections occurs rapidly, i.e., typically in less than two seconds. Once the internal pressure is withdrawn from the mold cavity, plates on the upper and lower portion of the molding apparatus retract away from the molded article in a horizontal direction. The mold sections then move in a vertical direction away from one another and the article is removed from the mold apparatus.
In some instances, the parison forming the lip buckles and then solidifies. This causes undesirable stretching in certain areas of the lip and reduces the strength of the lip. The present invention provides the capability for producing an article with a sturdy reverse lip without undesirable stretching, which lowers the mechanical strength and integrity of the lip.